Caffeine-tannin complexes

Most alkaloids exist in nature not in their free-base form but rather as the salt of naturally occurring acids known as tannins, a group of phenol-based organic acids that have complex structures. The alkaloid salts of these acids are usually much more soluble in hot water than in cold water. The caffeine in coffee and tea exists in the form of the tannin salt, which is why coffee and tea are more effectively brewed in hot water. As Figure 12.19 relates, tannins are also responsible for the stains caused by these beverages. 

Tea leaves consist primarily of cellulose this is the principle structural material of all plant cells. Fortunately, the cellulose is insoluble in water, so that by using a hot water extraction, more soluble caffeine can be separated. Also dissolved in water are complex substances called tannins. These are colored phenolic compounds of high molecular weight (500 to 3000) that have acidic behavior. If a basic salt such as Na2C03 is added to the water solution, the tannins can react to form a salt. These salts are insoluble in organic solvents, such as chloroform or dichloromethane, but are soluble in water. 

Another problem, posed by the beverage tea, is that in some cases persons who consume high quantities of tea may show symptoms of Vitamin B1 (thiamine) deficiency. It is suggested that the tannins in the tea may complex with the thiamine, rendering it unavailable for use. An alternative suggestion is that caffeine may reduce the levels of the enzyme transketolase, which depends on the presence of thiamine for its activity. Lowered levels of transketolase would produce the same symptoms as lowered levels of thiamine. 

The most notable feature of hydrolyzable and condensed tannins is to bind proteins, carbohydrates, and certain nitrogen-containing organic bases (e.g. caffeine, alkaloids). The phenomenon of protein complexation has been known for centuries and forms the basis for tanning leather with plant extracts (Sect. 10.3.3). Binding with polysaccharides is a less familiar property, but may well be equally important from a biological and commercial standpoint. 

A new dimeric plant tannin named woodfruticosin A, which is active as an inhibitor of DNA topoisomerase II, has been isolated from a Nepalese medicinal plant. Chelation with the ester groups at 0-1 and 0-6 of penta-O-galloyl- -D-glucose has been suggested as the mode of its complexation with various aromatic substrates (e.g., caffeine, daunomycin, methylene blue). -Pentagalloylglucose has also been reported to specifically inhibit two NADH reductases from E. coli. Two-dimensional H- and C-n.m.r. studies of natural polyphenols, with emphasis on the ester linkages between the phenolic adds and... 

Theobromine caffeine Epioatechin catechin Prooyanidins Complex tannins... 

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